U.S. Pat. No. 2,813,810 by Smith et al. discloses a method for fractionating invert sugar into glucose and fructose. In the Smith et al. method, the fructose of invert sugar is converted to diacetone fructose by acid catalysis with a commercial sulfonated phenol-formaldehyde ion exchange resin. After separating the dextrose precipitate and resin, the concentrated alpha-diisopropylidene D-fructose supernatant solution was hydrolyzed with 0.01N sulfuric acid at an elevated temperature for 12 hours. The hydrolyzate was neutralized by passing the syrup through an anion exchange resin. Crystalline fructose was recovered by dissolving the hydrolyzed concentrate in hot absolute ethanol and its crystallization therefrom by cooling.
In a paper authored by K. Erne ("Studies of Glycosides and Isopropylidene Derivatives" Acta Chemica Scandinavica 9, (1955), pages 893-901), acetone and fructose were catalyzed into 1,2-4,5-diisopropylidene D-fructopyranose. Erne observed that fructose readily converted into a brown, degradation product upon exposure to acid catalysts. Tipson et al. ("Acid-catalyzed hydrolysis of isopropylidene acetals of some 2-pentuloses and 2-hexuloses" Carbohydrate Research 10 (1969) pages 181-183) later reported that the acid-catalyzed hydrolysis of diacetone fructose with strong mineral acids (e.g. hydrochloric and sulfuric acids as disclosed by Erne, Smith et al. and others) produced high levels of decompositional by-products of fructose. Tipson et al. found that catalysis with 100 mM oxalic acid for 1-2 hours at 65.degree. C. would substantially reduce the level of fructose degradation.
Fructose may be obtained from a variety of natural and synthetic sources. High fructose corn syrups (HFCS) are conventionally manufactured by enzymatically isomerizing high dextrose conversion syrups. The enzymatic isomerization of dextrose syrups can provide fructose syrups which compositionally contain from about 30%-52% fructose, 40%-54% dextrose, 1%-4% disaccharide and from about 3%-8% saccharides of a D.P..sub.3 or higher. Since fructose is sweeter than dextrose, it is conventional to enrich the fructose content of a HFCS (e.g. 55%-90% or higher) by chromatographic fractionation and separation techniques. Food-grade syrups must necessarily be essentially free from organoleptically detectable by-products, which are often detectable in trace amounts. The elution of the fractionated monosaccharides under conventional enrichment processes leads to substantial dilution of the eluted products with water. Substantial capital equipment investments, evaporating, recycling, quality control and other manufacturing expenses are incurred under conventional fructose enrichment processes. It would be desirable to manufacture food-grade, 55%+HFCS syrups by a technique other than chromatographic fractionation and separation.
The laboratory studies of Erne, Smith et al. and Tipson et al. cannot be effectively applied to the fructose enrichment of HFCS. Many food applications for the enriched HFCS necessitate that the product be free from objectionable and organoleptically detectable flavor, color and other degradative by-products. Tipson et al. recognized substantial degradation of fructose arose as a result of the mineral acid hydrolyzing conditions of Smith et al. and Erne. Tipson et al. proposed to alleviate this problem by replacing the hydrolyzing mineral acids with a toxic acid (oxalic acid) which would be unacceptable for the manufacture of food products.
The acetonation of fructose is generally capable of yielding two isomeric forms of diacetone fructose. In modern practice, these isomers are generally referred to as 1,2:4,5 di-O-isopropylidene beta-D-fructopyranose and 2,3:4,5 di-O-isopropylidene beta-D-fructopyranose. The 1,2:4,5 isomer is generally prepared under kinetically controlled conditions whereas the thermally stable 2,3:4,5 form will generally require higher temperatures, higher concentrations of acid catalyst and longer reaction periods.
Within recent years, perfluorinated exchange resins with functionally active acid (e.g. sulfonic and/or carboxylic groups) have gained prominence for a variety of industrial applications. The commercially available perfluorinated ionic membranes are reportedly produced by a variety of chemical processes as disclosed in C&EN, Mar. 15, 1982 "Electrolytic cell membrane development surges" by S. C. Stinson, pages 22-25.